1. Field of the Invention
The present invention relates to an oily lubricant for cold plastic processing of metallic material, and particularly to an oily lubricant used for cold plastic processing, such as drawing, contraction of area, and forging of metallic material.
2. Prior Art
For cold plastic processing, such as drawing, reduction in area (drawing), and forging, of metallic material, various forms of lubricants are added in order to prevent frictional damage (burning) of the metal mold, die. etc. and the material to be processed, such as steel pipe and steel sheet, and to improve the quality of the processed product as well as to deter the wear of the tools.
In conventional drawings oils and press oils as the oily lubricants, mineral oils or synthetic lubricating oils are used as the base oil, with oiliness improvers and extreme pressure additives added to them. However, they are low in burning resistance; and when the draft becomes high in degree and the contact area between the material to be processed and the tool is increased, they tend to cause film breakage with resulting burning. Thus, they have been applicable only to relatively light degree processing (for example, sinking), and it has been totally unfeasible to use them for steel pipe drawing, etc. under the severe processing conditions. Also, since they contain the extreme pressure additives, metal corrosion is caused.
In conventional oily lubricants, for the synthetic lubricating oil as the base oil, polybutene, .alpha.-olefin, oligomer, polyethyleneglycol, etc. are used, and for the oiliness improver, fats and oils, saturated and unsaturated fatty acids with 10 or more carbons, fatty acid esters, phosphate, alcohol, etc. are used, and also, for the extreme pressure additive, chemical compounds containing sulfur or chlorine are used, respectively.
The base oil is a carrier that has the function of dissolving the oily lubricant and the extreme pressure additive homogeneously and stably and to feed them into the portions needing to be lubricated. Mineral oils and synthetic lubricating oils do not have strong polar groups and they are low in lubricating ability. Accordingly, they cannot be used as the lubricant by themselves, but chemical changes such as oxidation and deterioration are caused less to them, and because of that reason, they are used as the base oil.
The oiliness improver is the stuff that is fed by the base oil to the friction surface of metal needing the lubrication, and to cause the lubricating effect by being adsorbed. Fats and oils, fatty acids, etc. are long chain compounds with heavy molecular weight, and they have a strong polar group at one end of molecule. By the action of that polar group, their molecules are strongly adsorbed to and configured along the metal surface, thereby forming the adsorption film. Furthermore, because of the mutual attraction between the long molecular chains configured to make the adsorption film, the film becomes firm and strong, and shows the lubricating effect (to reduce the friction). However, as the temperature goes up, the configuration of molecules become disordered, and at about 150.degree. C., the film separates, and the lubricating effect is lost. Since the oiliness improver loses its lubricating effect when the temperature reaches 150.degree. C. or above, the adding of the oiliness improver under severe lubricating conditions is not effective. Consequently, an extreme pressure additive that shows the lubricating effect at high temperature is added.
The extreme pressure additive is the stuff that is supplied, same as the oiliness improver, to the metal's friction surface needing the lubrication, by the base oil, and forms the film on the metal surface, by chemically reacting with the metal when the degree of plastic processing becomes high and the contact area between the material to be processed and the tool becomes increased with the resulting extreme pressure condition which accompanies high temperature and high pressure. Through, the formation of the film mentioned above, the extreme pressure additive prevents the burning by reducing the friction, and improves the burning resistance of the lubricant. Chemical compounds containing chlorine or sulfur are different in reaction temperature range, respectively. Therefore, they are added in accordance with their purpose and use, when used together with the base oil and the oiliness improver. For the extreme pressure additive containing chlorine, chlorinated paraffin is used in many cases, and at 150.degree. C. to 250.degree. C., the C--C1 bond is cut off due to thermal decomposition, and Cl.sub.2 or HCl is formed. Thus, the extreme pressure lubricating effect is shown. However, such extreme pressure additive has a disadvantage that, when water is present, hydrolysis is caused, and HCL is liberated, thereby causing serious corrosiveness. The boundary lubrication temperature range of the sulfur compounds is about 250.degree. C. to 350.degree. C.
Also, as the conventional oily lubricants, there are those containing, in addition to the abovementioned three types of components, solid lubricants, such as graphite and molybdenum disulfide are mixed in them in order to improve the burning resistance. However, such oily lubricants tend to cause the burning, and the same as the other oily lubricants, they can be used only for the relatively low degree processing work (for example, sinking). Besides, such oily lubricants have problems including that the oil supply becomes difficult because of the presence of the solid lubricating agent, that they accumulate in the metal mold, or that they worsen the maintenance.
As has been described above, in the use of the oily lubricant, by means of the base oil, the oiliness improver and the extreme pressure additive are supplied to the friction surface of metal and adsorbed there. Then, in the temperature range from room temperature to about 150.degree. C., as the fluid lubrication range in which the oiliness improver serves to maintain the oil film, the oiliness improver shows the lubricating effect; while in the boundary lubrication range with higher temperature and with severe processing condition, the extreme pressure additive functions to maintain the oil film and shows the lubricating effect. However, as to the degree of the actual lubricating effect of the oily lubricant, because of the low burning resistance, it is only high enough for low degree plastic processing for steel pipe drawing, etc.
However, the oily lubricant can be used by merely supplying it to the surface of metal to be processed. Therefore, the process is simple and also it is quite easy to remove it after the processing.
Accordingly, various methods for improving the burning resistance of the lubricant as mentioned below have been employed conventionally.
For example, for cold drawing of steel pipe, the following methods have been used.
a. The steel pipe is treated with chemical conversion coating (for example, phosphate coating, oxalic acid oxidation coating) in advance, and also, provided with secondary lubricant (for example, metallic soap, such as sodium stearate, and oil).
b. The steel pipe is coated with a liquid form synthetic resin (liquid obtained by emulsifying a synthetic resin, or by dissolving the synthetic resin in a solvent) in advance, and the coating is solidified by drying.
Also, for the drawing of the sheet form metallic material, the methods as mentioned below have been used.
c. The lubrication film is thickened by directly coating the material with press oil that is specifically made high is viscosity.
d. Press oil with extreme pressure additive added to it is used for directly coating the material with it.
e. After coating the material with a solution of synthetic resin, such as vinyl chloride resin, hardening by drying is carried out.
f. The synthetic resin film is directly adhered to the surface of material to be processed.
However, these methods are not satisfactory in terms of simplicity in lubricating treatment, readiness for removal after processing, and the clearness of the metal surface. Practically speaking:
The method a. mentioned above enables the lubricant to show sufficient lubricating effect even when the degree of processing (draft) is high. However, because the lubricant is of the chemical reaction type, it is troublesome to control the processing solution; and also because the life of the lubricant is short, it is necessary to carry out waste solution disposal treatment frequently. Thus, this method has an economic problem, and at the same time, it may become the cause of the environmental pollution. Besides, since the lubrication film adheres too well, it is difficult to remove the film from the product after the processing. Furthermore, since the processing is of the chemical reaction type and if the material is highly anticorrosive, the chemical treatment itself is hard to carry out, and the homogenous chemical conversion coatings cannot be formed.
The method b. is to physically adhere the film over the surface of the metallic material, and a lubricant having lubricating characteristics superior to those obtained by the method a. are developed. However, it has a disadvantage in that it requires extensive drying to obtain the lubrication film. The removal of the film after the processing becomes more difficult as the lubrication film becomes stronger.
The method c. is defective in that the processability becomes lower with increase in viscosity; and the method d. is disadvantageous in that, since it uses a somewhat restrained corrosion reaction, severe corrosiveness is caused by the presence of water, etc.
The methods e. and f. make it possible to perform drawing of a high degree in draft and deep drawing which are not achievable by the drawing oil and press oil. However, they have the shortcomings in that they require strenuous work for drying and adhesion of the resin film, and also in that they are considerably higher in cost in comparison with the drawing oil and the press oil.
As has been mentioned above, while conventional lubricants have characteristic effects, respectively, every one of them has some problems.